Vibration-type measuring devices of the generic type are also referred to as Coriolis flow sensors. They can be used to measure the flow rate of a fluid medium flowing through the measuring tube. Other parameters of the medium, for example the viscosity or the density of the medium, can also be measured either in addition to the flow rate or on their own.
A Coriolis flow sensor has a frequency characteristic with a greatly pronounced resonant frequency ω0 or f0 (it is known that the connection between ω0 and f0 is given by the relationship ω0=2π×f0) in the balanced mode. The measurement variable can be determined with excitation at this resonant frequency since the amplitudes of the measured signals are at a maximum at the resonant frequency.
However, during actual operation of the Coriolis flow sensor, the resonant frequency f0 is shifted on account of external and internal influences, primarily with the density ρ of the medium inside the measuring tube. Furthermore, the resonant frequency f0 can also change as a function of the temperature and other environmental conditions.
Therefore, Coriolis flow sensors have control and evaluation electronics whose aim is to always excite the system at the resonant frequency f0.
In one application, as shown in DE 10 2007 059 804, for example, the excitation is effected by means of torsion in the centre of the measuring tube.
Sensors (S1 and S2) which register the deflections of the measuring tube and thus record the excitation frequency f are used to measure the phase shift of these points with respect to the excitation and are symmetrical with respect to the excitation (upstream and downstream). The condition for resonance of the system is achieved as soon as the current through the actuator Iactuator and the average phase shift of the sensor signals are in phase. The current through the actuator can be a measure of the excitation force in the plunger-type armature sensors, which are often used as the actuator.
The actual measurement variable of the Coriolis flow sensor is the phase shift f between the sensor signals S1 and S2 (upstream and downstream of the excitation) during excitation at the resonant frequency f0. This signal corresponds to the mass flow rate (m′) of the Coriolis flow sensor.
If another measurement variable, for example the viscosity of the measurement medium, is intended to be detected, only one sensor is required under certain circumstances. In that case, the attenuation of the sensor signal is measured, for example, and is used to determine the viscosity, or else the change in the phase angle of the one sensor signal based on the exciting force could be measured and could then be used to determine the viscosity or else the density.
However, the common feature of all measurement methods nowadays is a desire to measure at the resonant frequency as far as possible.
In order to track the resonant frequency in the event of a change in the system, a control loop in known Coriolis flow sensors controls the excitation frequency until the resonance condition has been satisfied, that is to say until the average value of the phases of the sensor signals is in phase with the excitation current. In the case of a viscosimeter having only one sensor, the frequency would accordingly be controlled until the phase of the one sensor signal were in phase with the excitation current. For this purpose, the frequency must be varied until the phases of the signals satisfy the condition.
However, this operation requires a certain amount of time on account of internal mechanical compensation or relaxation processes. If a new frequency is applied, the device still oscillates for a time with the phase associated with the old frequency and it takes a certain amount of time before this “shifted component” of the mechanical oscillation has disappeared as a result of attenuation. It is thus necessary to wait for some time before the new phase associated with the new frequency can be reliably measured; to be precise, the longer the wait, the better the quality of the system. The speed at which the natural frequency can be tracked is thus limited by the mechanical relaxation of the system.